Expression of Heterologous Proteins in Plant Seeds
The expression of heterologous proteins in plant seeds offers the possibility of, for example, producing large quantities of easily harvested polypeptides, and of expressing proteins that improve their grain quality. Discussions of this concept can be found in U.S. Pat. No. 5,714,474 (“Production of enzymes in seeds and their uses”).
Hordein Storage Proteins
Barley seed storage proteins account for about 8 to 15% of the dry weight of the mature barley grain. The major seed storage proteins in barley are alcohol-soluble prolamines, termed hordeins, which are classified into two major groups, B and C, and two minor groups. D and γ (Shewry 1993). Depending on nitrogen levels, these four groups account for about 35 to 55% of total barley seed protein. The B- and C-hordeins account for about 70 to 80% and 10 to 20%, respectively, of the total hordein fraction, with small amounts of D- (2–4%) and γ-hordeins (not precisely determined). The B-, D- and γ-hordeins are sulfur-rich prolamines while the C hordeins are sulfur-poor prolamines (Bright and Shewry 1983). The hordeins are coordinately synthesized in the developing starchy endosperm tissue (Giese et al. 1983; Sørensen et al. 1989). They are cotranslationally transported into the lumen of the rough endoplamic reticulum, with simultaneous cleavage of the signal peptide, and are ultimately deposited into protein bodies (Cameron-Mills 1980, Cameron-Mills and von Wettstein 1980; Cameron-Mills and Madrid 1989).
Genetic analyses show that all hordeins are encoded by structural genes on chromosome 5 (1H) of barley; the Hor1, Hor2, Hor3 and Hor5 loci on chromosome 5 encode the C-, B-, D-, and γ-hordein polypeptides, respectively (Jensen et al., 1980; Shewry et al. 1980; Blake et al. 1982; Shewry et al. 1983; Shewry and Parmar 1987). The genes for B-, C- and D-hordeins have been isolated and characterized (Brandt et al. 1985; Forde et al. 1985; Rasmussen and Brandt 1986; Sørensen et al. 1996). The B- and C-hordeins are encoded by multigene families comprising 10 to 20 members while D-hordein is encoded by a single gene (Brandt et al. 1985; Rasmussen and Brandt 1986; Sørensen et al. 1996). The regulation and expression of these hordein promoters have been studied by transient expression assays (Entwistle et al. 1991; Müller and Knudsen 1993; Sørensen et al. 1996) in barley endosperm. As determined by these assays using promoter-uidA fuisons, the D-hordein promoter is 3- to 5-fold more active than the B- or C-hordein promoters tested (Sørensen et al. 1996). The B-hordein promoter has also been studied using stable tobacco transformation with promoter-cat fusions (Marris et al. 1988).
Although the genes for B-, C- and D-hordeins have been isolated and characterized, their regulation and expression have been studied only in transient expression assays in barley and in stably transformed tobacco (Brandt et al., 1985; Forde et al., 1985; Marris et al., 1988 Sørensen et al., 1996).
In barley, wheat and maize, the major highly insoluble prolamin storage proteins are synthesized on polysomes closely associated with the endoplasmic reticulum (ER). (See Seeds Physiology of Development and Germination. 2nd ed., eds. Bewley and Black Plenum Press, New York 1994). Newly synthesized proteins pass through the ER membrane into the lumen where they aggregate into small particles, which eventually form larger aggregates and protein bodies (which can be observed in electron micrographs).
In wheat, two different types of protein bodies accumulate independently within the developing endosperm: low density bodies that develop earlier and high density bodies that develop later and are derived from the ER. The high density proteins are formed when aggregation of proteins inside the lumen of the ER puts a strain on the membrane and cause it to rupture. The membrane may reform free of the protein aggregate, after an interval in which the protein body itself is not bounded by a membrane. In other cereals besides wheat and barley, such as millet, rice, maize and sorghum, the protein bodies remain as distinct membrane-bound entities even in mature seed.